Howdy all. I have a model 31F777 17HP (busted rod) and 31F707 18HP (flat intake cam lobe) that I mixed and matched parts to make the 31F707 18HP run. I checked all part numbers on B&S website prior to starting this project and all numbers where the same. Basically this is what I did, I used the cam from the 17HP and installed it in the 18HP. The ACR was broken on the 17HP so I robbed it off the 18HP, along with the stator and flywheel cuz I needed that for the electric PTO. Installed 18HP in mower and fired er up, easy enough start up, ran great at high rpms, but backfired bad at low rpms. Rechecked my valve adjustment, still within specs, rechecked flywheel key way, not sheared, cleaned carb, not dirty, changed spark plug, no change, also sprayed carb cleaner all around intake with no changes. So then I brought piston up to TDC, held flywheel bolt with wrench, and with my rubber tipped air nozzle pumped approx 100 psi of air in cylinder. It yanked the wrench out of my hand pretty quick and jammed my finger at the same time, ouch :confused2: So I repeated this process again with a firmer grip and did not hear any air escaping, anywhere. Hmmm... So at this point I pulled the head and inspected valves thinking that one of the guides dropped, not the case and both valves have good seats and aren't sticking. Then I checked the head gasket, blown at weak point at push rod side, Hmmmm.. why didn't it leak when I pumped air in the cylinder? So I clean er all up, install new head gasket, adjust valves, etc, and fire er up again. Same results, backfiring through the carb. :frown: What I'm thinking now is that maybe the ACR is not functioning properly and holding the intake open. I did operate the ACR 1/2 dozen times prior to install and it seemed OK. If anyone knos where to look next, please let me know. Thanks again all :thumbsup: